First, give the Brits all Oregon. Pick your reason, a weak US, early civil war, for some reason they need to sell it, etc. Butterflies see that William Seward is off doing something different, and the US never buys Alaska. Come Crimea, the Brits and western *Canadians grab on and don't let go. Let's say early Yukon gold, get some more people spread all along the upper west coast earlier. With a stronger and more heavily populated west with an even more unique character, it's "Nuts to your Confederation!" and eventually newly formed Canada and Columbia split Rupert's Land between them. For a vigorous young country with a vigorous young leader, Prime Minister C. Alan Arthur, the future is bright!
What's that? A shitty visual aid, you say?
There we have it. Obviously, the US wouldn't have such OTL-esque borders, and the Columbia/Canada border would likely not hew so close to OTL's provincial borders. Nevertheless.
I kinda like this. Sticking a bookmark in this idea for later.